The present invention relates to a sealing device for using heat to form a weld between a first layer of plastic and a second layer of plastic, provided with heating means for heating the layers of plastic and conveyor means for conveying the layers of plastic past the heating means, and separation means for forming a separating layer between the heating means and the layers of plastic while the latter are being heated, the separation means being displaceable in the direction of movement of the layers of plastic past the heating means.
A device of the type which is described in the preamble is known from Dutch patent NL 1015127. The sealing device according to the prior art is provided with heating means which are designed as a sealing wire. This sealing wire is held taut between two stationary sealing elements which are responsible for supplying current to the sealing wire. The sealing wire is made from resistor material and will become hot when current is passed through it. The width of the sealing wire is a few millimetres. This width corresponds to the width of the weld which is to be formed between a first layer of plastic and a second layer of plastic. Beneath the sealing wire, there is a support block allowing pressure to be exerted on the layers of plastic which are guided through beneath the sealing wire.
In use, two layers of plastic are guided through beneath the sealing wire. The heat which is introduced into the layers of plastic will cause the plastic to at least partially melt, so that bonding can take placed between the first and second layers. If no further measures were to be taken, the molten plastic material could stick to the sealing wire. Therefore, what are known as separation means are described in NL 1015127. These separation means are designed as a teflon strip which is guided in an endless loop around the heating means by the device. In use, the speed of movement of the teflon strip is the same as that of the layers of plastic which are moved through beneath the sealing wire, i.e. the teflon strip is stationary with respect to the plastic layers and moves over the sealing wire. The teflon is able to withstand the relatively high temperature of the sealing wire and will not melt as a result of the introduction of heat. The presence of the teflon strip substantially avoids the problem of possible sticking of the plastic material to the sealing wire or to the sealing elements between which the wire is held taut.
According to the prior art, the plastic belt or strip is guided underneath a sealing wire via guide rolls. After it has left the downstream section of the sealing wire, the teflon strip will be guided back to the upstream start of the sealing wire via the rolls. While it is being conveyed, the teflon strip will cool and will return to ambient temperature. From initial contact between the sealing wire and the teflon strip, the teflon will have to be heated from ambient temperature until the temperature of the teflon is the same as the temperature of the sealing wire itself. Then, the heat is transferred to the plastic below it by the teflon. It is desirable to keep the sealing wire as short as possible, i.e. within a relatively short distance it is necessary both to heat the teflon and to supply heat to the plastic layers below it. Therefore, a relatively high temperature of the sealing wire is used. Since a relatively high temperature is used, what is known as a hot spot will occur at the downstream end of the sealing wire. Both the teflon and the plastic below it are heated with the aid of the sealing wire over the downstream section. The sealing wire cannot readily release heat at the location of the hot spot. This leads to the sealing wire being subjected to thermal loads at the location of the hot spot and to the teflon material and the plastic layers below it being excessively heated at this location. Furthermore, dust and dirt will bake on at the hot spot. This dirt is relatively sharp. The teflon strip which runs passed this sharp dirt will as a result become worn more rapidly. This represents a drawback.
In view of the drawbacks of the prior art, the object of the present invention is to provide a device of the type described in the preamble in which the above drawbacks are avoided as far as possible.
According to the invention, this object is achieved by the fact that preheating means are provided for the purpose of preheating the separation means before they are fed to the heating means.
According to the invention, it is provided that the separation means, which are designed, for example, as a teflon belt or strip, are preheated at a section which lies upstream of the heating means. When the layers of plastic, together with the separation means, reach the heating means, the separation means are already preheated. This means that the heat which is fed to the separation means and the layers of plastic by the heating means can pass directly to the plastic layers themselves, i.e. it is possible to use a sealing wire which is at a relatively low temperature. The preheating of the separation means means that there is no need to supply excessive heat to a starting section of the heating means, i.e. a hot spot at the end of the heating means is also eliminated.
According to the invention, it is possible for the heating means to form a guide for the separation means, in which case the heating means comprise a first section for preheating the separation means which adjoins a second section for heating the layers of plastic. In this case, it is possible for the preheating means to comprise a first section for bringing the separation means up to temperature, which first section adjoins a second section for further increasing the temperature of the separation means.
According to these measures, it is possible to design the heating means as a simple sealing wire, with the separation means being guided past this sealing wire. The sealing wire may, for example, have a first section which is at a relatively low temperature and adjoins a section which is at a higher temperature, i.e. the separation means, such as for example a strip of teflon, are gradually heated from room temperature towards the temperature of the sealing wire itself. When the separation means together with the plastic reach the heating means, the separation means have been heated to such an extent that they are at a temperature which substantially corresponds to that of the sealing wire itself.
According to the invention, it is possible for the separation means to form a belt or strip which can be displaced along the heating means. In this case, it is possible for the belt or strip to comprise teflon. Furthermore, according to the invention it is possible for the belt or strip to be of endless design.
As has already been stated above, it is advantageous for the heating means to be designed as a resistor.
In this case, it is possible for the resistor to have a first section having a first resistance and a second section having a higher resistance, in order, in use, to form a first section which is at a relatively low temperature and a second section which is at a relatively high temperature.
According to the invention, it is furthermore provided that the resistor has a core of resistor material, the first section being coated with a conductive material in order to reduce the resistance at the location of the coating. According to this embodiment, the heating means are designed as a sealing wire comprising, for example, resistor material. The first section of this sealing wire is coated with a material which reduces the resistance of the material. This coating means that the resistance will be lower, and the temperature which is to be generated when current is passed through will also be lower.
According to the invention, it is possible for the coating to comprise nickel or copper.
A further preferred embodiment is characterized in that the device comprises first heating means and second heating means, which heating means are positioned opposite and at a distance from one another, so that a passage for the layers of plastic which are to be joined is formed between them. In this case, it is possible for one of the heating means to be displaceable from an active position into an inactive position.
As has already been stated above, according to the prior art it is customary for the layers of plastic which are to be sealed to be guided past heating means which run substantially parallel to a support which is used to exert pressure. A certain pressure can be exerted on the layers of plastic when they lie between the support and the heating means. According to the invention, there is provision for the support or pressure-exerting means itself to form a second heating means, i.e. for the heat to be supplied to the layers of plastic which are to be joined from both sides.
The fact that one of the heating means can be pivoted away from an active position into an inactive position means that it is possible, for example, to guide layers of plastic between the two heating elements without the heating elements being switched on.
According to the invention, it is also possible for the heating means to be designed as a strip or wire which is secured to a base surface, the wire projecting out of the base surface and the projecting section of the wire being smaller than the overall thickness of one of the plastic layers.
If the device according to the invention is used to process plastics, such as HDPE and LDPE, temperatures which cause the material to melt are generated in the vicinity of the sealing wire. The molten plastic has a low viscosity and is therefore unable to withstand compressive force. If the sealing wire which is to be used were to project too far out of a base material, the sealing wire could start to act as a blade. To prevent this, it is desirable, according to the invention, for the projection of the sealing wires to be limited, so that the material cannot be completely cut through.
According to the invention, it is also possible for the conveyor means to be arranged downstream of the heating means. In this context, it is possible for the conveyor means to comprise pressure-exerting rollers which are arranged in line with the heating means.
The layers of plastic which are to be sealed are moved past the heating means with the aid of the conveyor means. These conveyor means are, for example, in the form of plastic pressure-exerting rolls which are positioned downstream of the heating means. Use of pressure-exerting rolls which are positioned in line with the heating means enables these conveyor means to apply a certain amount of pressure to the freshly formed weld.
Furthermore, according to the invention it is possible for the device to have a supply path for the layers of plastic, which supply path comprises a turn or bend upstream of the heating means.
When a first layer of plastic is being sealed to a second layer of plastic, there is a risk of creases forming in the layers of plastic themselves. Creases of this type are detrimental to the quality of the weld which is to be formed. If the supply path upstream of the heating means is now provided with a turn or bend, it is possible to allow the layers of plastic to crease deliberately on the side which is not being sealed, so that the opposite side, which is to be provided with a seal, is automatically pulled taut.
According to the invention, it is possible for the layers of plastic to be supplied from a storage holder or roll, via a guide, the longitudinal axis of the guide forming an angle of less than 90xc2x0 with the longitudinal direction of the heating means.
This measure ensures that the side which is to be sealed is held taut while a crease is automatically imposed on the opposite side.
According to the invention, it is advantageous for the angle between the longitudinal direction of the heating means and the axis of rotation to be substantially 85xc2x0.
Furthermore, the present invention relates to a sealing device which is characterized in that the device comprises a spindle on which a roll of plastic can be placed, which spindle has a first projecting end and a second end, with the aid of which the spindle is secured to the device, the spindle being provided, at the first end, with a conically shaped member with a diameter which increases from the first end towards the second end, and the spindle also having, at a distance from this end, a second conical member with a diameter which increases, as seen from the first end towards the second end, a section of the spindle of relatively small diameter being present between the two conical members.
If, by way of example, a storage roll is to be placed onto the spindle according to the present invention, the core of the storage roll is pushed over the first conical member. The conical member ensures that the roll is guided in the correct direction. Then, the roll can be pushed on until contact is made with the second conical member. This conical member may be designed in such a way that it comes to be fixed in a clamped manner against the inner side of the roll which has been positioned on it. Furthermore, the spindle will comprise a stop, in which case the distance over which the storage roll can be pushed onto the spindle can be defined by this stop.
Furthermore, the invention relates to a sealing device which is characterized in that means are provided for feeding air between the layers of plastic which are to be joined, which means comprise an outlet opening, the outlet opening decreasing in size in the direction of movement of the layers of plastic.
The sealing device according to the present invention is particularly suitable for sealing a prepared film. This film comprises a tubular film in which transverse seals are made. These seals extend from a first end side to a distance from a second end side. A tube of this type is guided into the device according to the present invention. The space which remains between the end of the transverse seal and the second end side of the film can be used to slide the film over a guide member. At the guide member, there is a blade for cutting the tube open at this location. Furthermore, the device includes pressurized-air means for allowing air to be blown between two successive transverse seals. Blowing in air will cause the tube to adopt a three-dimensional form and to bulge out to some extent. Then, the device according to the present invention is used to make a seal in the prepared material. To prevent the air which is blown in from escaping as far as possible, it is advantageous if the outlet opening of the pressurized-air means adjoins the sealing means as closely as possible. This can be achieved by using an outlet opening which decreases in size in the direction of movement of the layers of plastic which are to be joined, i.e. a relatively thin or narrow outlet opening can virtually immediately adjoin the heating means in order to form a seal between two layers of plastic.
In addition to the device described above, the present invention also relates to a method which is characterized in that the separation means are preheated before they are fed to the heating means.